It's not mandatory but the team that leads the general team classification usually wears a yellow helmet. If a rider has also won a national championship, they get to wear that countries jersey instead of the team uniform.

Sergio is wearinga yellow topped jersey, if you see the full jersey it is not all yellow. The reason is he is the Colombian National Road Champion so he gets to wear that jersey for all road events. The yellow helmets are for the whole team and they are wearing them because they are the leaders in the team classification.

For more on this topic though see this article that came out recently on Velonews: https://www.google.com/...a9_Gm0WDRhCyOAeRqTlH

Hugita's jersey is indeed very confusing when you see it.
Here is a article that explians it all
https://www.velonews.com/...arent-maillot-jaune/

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Just to add a bit more info on yellow helmet when EF isn't leading team classification: Higuita has been wearing the yellow helmet all season to match his jersey, Caicedo has been doing the same with the Ecuadorian Champs jersey. Here's Caicedo at Tirreno-Adriatico this week (extremely impressive pulling EF & Woods for his stage win):

I think it actually is mandatory now. The rule changed a few years ago iirc.

Higuita in a yellow helmet when the rest of his team are in pink helmets: